1. The magistrates and the police work very closely together. By nature of the relationship alone, they are likely to be inclusive of requests. When information about what is being done, as is alleged in this case, it effectively opens the police to access to anything.
Even assuming the agencies talk to one another, and for all the FBI's law-bending, they still have to present their evidence in court, and 'your honor, we got this off an NSA dragnet' isn't gonna cut it.
Heh, well, give it a few more years.
1) Obtain information about a crime via method you don't want made public (classic example is ongoing undercover work or ongoing wiretap)
2) Have someone tell a CI, then have CI report it back
3) use report to follow up and obtain sufficient justification for warrant
4) actual source of the tip is never disclosed in court.
Not saying that's what happened here because the NSA doesn't give two shits about some guy that filled out a few fraudulent tax returns, just pointing out that that shit never show up in court. That's also why very few espionage cases are brought to trial.